A battery mounted in an electric vehicle (EV) is expensive, and the ratio of the price of such a battery to the price of an electric vehicle is higher than the ratio of the price of a battery to an electronic device such as a notebook computer or a portable telephone. For example, some people point out that the biggest factor in the high costs of electric vehicles is the expensive batteries. Therefore, when electric vehicles are put on a secondhand car market, it is essential to know the present price of each battery.
However, the degrees of deterioration of batteries vary with the states of usage, and the values of those batteries cannot be calculated simply from the age of service or the number of charge and discharge cycles. Therefore, it is difficult to set the present price of each battery on a secondhand car market or the like.
To counter this problem, there have been various kinds of suggestions made about methods of determining the degree of deterioration of a battery.
For instance, methods of determining the degree of deterioration of a battery using the internal resistance of the battery have been proposed. In other proposals, the present capacity of a battery is determined from the cumulative value of charged/discharged current, and the ratio of the present capacity to the initial battery capacity is calculated to determine the degree of deterioration.
However, a battery's internal resistance components that affect the battery capacity and power are not only the DC resistance that is the resistance components of an electrolytic solution. Therefore, it is difficult to predict a future deterioration tendency by a determination method using only the DC resistance components.
Also, if the histories (the deterioration patterns) that indicate how batteries with the same battery capacities have deteriorated differ from each other, the deterioration states in the future will not necessarily be the same as each other. Therefore, a present value determined only from the degree of deterioration based on the battery capacity can be unreliable.
Normally, a battery to be mounted in an electric vehicle is required to show high performance such as a large current and a large capacity, compared with batteries to be used for other purposes. Therefore, batteries that have been used in vehicles but no longer satisfy those high requirements might still sufficiently fulfill other purposes.
Since batteries that are already useless in vehicles can be useful for other purposes, the present price of each battery needs to be determined by taking into consideration the purpose of future use of the battery.
As described above, there is a demand for a method of determining the deterioration state of a battery with high precision and a rational indicator of the present value of the battery according to the purpose of future use.